The Effects of H₂SO₄ on the Mechanical Behavior and Microstructural Evolution of Polycrystalline Ice

Abstract

The Earth's large continental ice sheets contain a variety of naturally occurring impurities, both soluble and insoluble. Understanding how these impurities affect the rheology, intrinsic thermodynamic properties, and fate of these ice sheets is not well understood. To investigate the effects that trace amounts of H2SO4 have on the flow and ductility of polycrystalline ice, a series of mechanical tests were conducted at −6, −10, −12.5, and −20°C using laboratory‐prepared specimens of polycrystalline ice doped with 1–15 ppm of H2SO4. Parallel tests were performed on identical but undoped specimens of polycrystalline ice. Mechanical testing included constant‐load tensile creep tests at an initial stress of 0.75 MPa and compression tests at constant displacement rates with initial strain rates ranging from 1 × 10−6 to 1 × 10−4 s−1. It was found that H2SO4‐doped specimens of ice exhibited faster creep rates in tension and significantly lower peak stresses in compression, when compared to the undoped ice. Postmortem microstructural analyses were performed using cross‐polarized light thin section imaging, X‐ray computed microtomography, Raman spectroscopy, and electron backscatter diffraction. These analyses showed that H2SO4‐doped specimens had a larger grain size at strains ≤15%, and an earlier onset of microcracking at lower strain rates than the undoped ice. Strain‐induced grain boundary migration was found to be the predominant mechanism of recrystallization in both doped and undoped specimens. Further, an aqueous phase of H2SO4 was found to exist at the grain boundaries and triple junctions of the doped ice, which is thought to have significantly contributed toward its reduced viscosity.

Dynamic Observations of the Densification of Polar Firn Under Compression Using a Micro‐Computed Tomograph

“…These maximum stresses were comparable to the ultimate compressive stress of 1.68 MPa obtained in uniaxial compression tests on snow samples with densities from 350 to 600 kg m −3 at temperatures of −25°C to −5°C and different strain rates by Wang et al (2021). However, these maximum stresses were much lower than the peak stresses of ∼5.4 and ∼4 MPa for ice specimens at strain rates of 1 × 10 −4 s −1 and 1 × 10 −5 s −1 at −10°C (Hammonds & Baker, 2018). Thus, it is plausible that the MaxStr of 0.35-2.1 MPa varied with varying density in our tests.…”

The temperature change shortcut: effects of mid-experiment temperature changes on the deformation of polycrystalline ice

“…Strain rates at the primary, secondary and tertiary creep stages are controlled by the specific stress and temperature conditions, as well as microstructural and chemical characteristics of the ice (Gao and Jacka, 1987;Treverrow et al, 2012;Hammonds and Baker, 2018). Because ice in many natural scenarios has been flowing for some time and typically has already reached a quasi-constant tertiary strain rate (except in some key regions where assumptions of tertiary creep are not valid (Budd et al, 2013;Graham et al, 2018)), the accelerating and tertiary creep stages are of interest to many glaciologists (Gao and Jacka, 1987).…”

Microstructural characterization of snow, firn and ice

“…The micro-CT can also be used to show the distribution of dust particles, tephra and air bubbles in specimens taken from ice sheets and glaciers as long as they are greater than 5 µm. The micro-CT can also be used to image cracks in ice [23,69]. Unfortunately, the micro-CT cannot be used to image the individual ice crystals since the X-ray attenuation is independent of grain orientation.…”